Most animals that don't live as long as we do are also smaller than we are. A lot of things are proportional to how big an animal is, and scientists think it has something to do with how our bodies use oxygen to 'burn' the food we eat. I think bigger animals, such as elephants, live longer than we do. Scientists are still trying to understand all this. Pigeons and rats are the same weight, but rats live about 4 yrs and pigeons live about 40 yrs! Scientists have found that rats produce toxic chemicals ("radicals") from oxygen 10 times faster than pigeons do, so they think this is why rats die off faster. Also, scientists found that animal cells all live about the same length of time when they're grown in petri dishes in the lab, whether they come from a big animal with a long lifespan or a little animal with a short lifespan. It's a complicated question.